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THE BLAST PUENAOE.                                       55
(4)   FeA+3 C=2 Fe+3 CO.
(5)  Fe304+4 0=3 Fe+4 CO,    '
(6)   FeO+C=Fe+CO.
Each of these reactions is endothermic—i.e., it absorbs heat.
The carbonic acid (C02) formed by the reduction of iron oxide by carbonic oxide (CO), or by carbon, is an oxidizing agent, and by a change in temperature there may be a reversal of the reduction just performed, according to the following reactions:
(7)   2 FeO+C02=Fe203+CO.
(8)   2 Fe+3 C02=Fe203+3 CO.
The first creating heat and the second absorbing energy. These reactions depend upon both the temperature and the dilution of the gas •with carbonic oxide. At high temperatures the action is strong, and considerable carbonic oxide must be present to avoid reoxidation. The main landmarks of the relations may be thus summarized:
(a) Carbonic acid (C02) begins to oxidize spongy iron at 300° C. (570° F.).
(b)   Carbonic acid (C02) begins to unite with carbon at 550° C. (1020° F.), and the reaction is complete at 1000° C. (1830° F.).
(c)   The reduction of metallic iron depends upon the percentage of carbonic acid (C02) in the gases, but the critical content of C02 depends 'upon the temperature, as follows:
At a white heat a gas containing C02=10%, C0=90%, will not reduce metallic iron from the oxide.
At a full red heat a gas containing C02=327o, CO—68%, will not reduce metallic iron.
At a low red heat a gas containing C02=60%, C0=40%, will not reduce metallic iron.
A mixture of C02=50%, CO—50%, passed over spongy iron at a white heat oxidizes it to FeO, while if passed over Fe203 reduces it to FeO.
The reactions in the upper part of the blast furnace are not simple processes of reduction like reactions (1) to (6), or oxidations like (7) and (8). While these actions are progressing there is a deposition of carbon, according to relation (9),
(9) 2 Fe203+8 C0=7 C02+4 Fe-f C.      .